The objective of this proposal is to identify factors and conditions that regulate estradiol synthesis in ovarian follicles. Estradiol is the dominant steroid product of developing preovulatory follicles and continued estradiol secretion is essential for follicles to reach ovulatory status. There is evidence for a number of species that both theca and granulosa cells are needed for estradiol production because the theca cells make androgen which is aromatized to estradiol by the grandulosa cells. It is believed that both FSH and LH are essential for follicular development and estradiol secretion. Although LH stimulates androgen production in many species, the roles of FSH and LH in regulating the conversion of androgen to estradiol are poorly understood, except in rats. This proposal is designed to investigate the roles of FSH and LH in steroidogenesis and follicle development in cattle and to examine the actions of intrafollicular steroids as potential regulators of follicular steroid production. Specifically, endogenous levels of gonadotropins will be altered at specific times during the bovine estrous cycle by injections of inhibin or an antiserum to LH. The effects on follicular development and regression will be assessed in vivo by ultrasonography of the ovaries and in some experiments follicles will be isolated and follicle cells cultured to determine the effects of altered endogenous gonadotropin levels on their steroidogenic capabilities. In other experiments the roles of androgen and estradiol as intrafollicular regulators of steroidogenesis will be explored by culturing theca and/or granulosa cells obtained from preovulatory follicles of cattle and rats with various concentrations and combinations of these steroids, in the presence and absence of gonadotropins. Finally, the relationship between follicular estradiol production and luteinization will be explored, using progesterone and oxytocin secretion and granulosa cell morphology as markers for luteinization. The proposed experiments will provide a better understanding of the actions of gonadotropins and steroids in the regulation of follicular estradiol secretion and thus, may eventually lead to the development of better methods of inducing ovulation and regulating fertility.